Mechanical parts productive of interpart friction are required to exhibit improvements in abrasion resistance and fracture toughness though both properties can not be improved easily. For example, in steel parts, high carbon steel can be controlled through quenching to provide an improvement in abrasion resistance. In that case, though, the toughness of the total material is lowered. If the toughness is taken into account and thus less-carbon content steel is quenched and tempered, the abrasion resistance is deteriorated conversely. As a method for satisfying two requirements, that is, improvements in abrasion resistance and toughness, a method for surface hardening, such as carburizing and nitriding, has been known, which imparts abrasion resistance to a surface layer while holding toughness inside the material, thus hardens only the surface. This surface hardening method, however, has a problem because steel parts can not be used easily at a high temperature of 1000° C. or higher.
On the other hand, SiC and Si3N4 have both excellent abrasion resistance and high-temperature characteristic and accordingly have been developed progressively for the use as abrasion-resistive materials at high temperatures. These materials, however, have brittleness, which is the drawback inherent in monolithic ceramic, and are very sensitive to intrinsic small pores or cracks. Therefore, even if quality control is executed thoroughly to produce ceramic parts with no defect, minute defects such as microcracks in use caused greatly lower the material strength as a problem. For example, when ceramic parts are exposed to an environment with a sharp variation in temperature, a thermal expansion difference between the surface and the interior yields an internal stress, which causes microcracks in the surface and fractures these defect-sensitive ceramics. Therefore, it is required to replace the parts with new parts frequently before fracture of the parts.
A ceramic fiber-reinforced, ceramic matrix composite (hereinafter referred to as CMC) has a higher fracture resistance than the above monolithic ceramics. The higher fracture resistance is resulted from a sliding layer on the fiber surface, which is formed to prevent cracks from developing to adjacent fibers. In the CMC, however, the sliding layer has no abrasion resistance function and accordingly the fiber and the matrix fall out in accordance with abrasion of the sliding layer in the fiber surface as a problem. For the purpose of preventing this, a coating of ceramic material excellent in abrasion resistance is applied (Patent Document 1).
Patent Document 1: JP 2006-143553A